Clojure summing numbers in a map, a short story

example.clj

(def fundings (sql/select-external-fundings sql/db { :year 2017 })) ;; raw data: ({...} {...})

(def sum-keys [:industry_collaborative :research_charity]) ;; keys we want to sum

(def sum-fundings map #(select-keys % sum-keys) fundings)) ;; collection with only keys we want to sum
;; => ({:industry_collaborative 1M :research_charity 2M}
;; {:industry_collaborative 1M :research_charity 1M}
;; {:industry_collaborative 10M :research_charity 1M})

(def start-map (reduce #(assoc %1 %2 0) {} sum-keys)) ;; map with keys all with values of 0

;; wanted end result:
;; {:industry_collaborative 12M :research_charity 4M}

(reduce #(assoc %1 :research_charity (+ (:research_charity %1) (:research_charity %2))) start-map sum-fundings)
;; => {:industry_collaborative 0 :research_charity 234.00M}

;; ^ works for hardcoded key
;; %1 is the return value of the last fn call, or start-map on the 1st call
;; %2 is an item from the collection

;; break it up...

 (defn summer [result item] (assoc result :research_charity (+ (:research_charity item) (:research_charity result))))
 (reduce summer start-map sum-fundings)
 
;; summer needs to take two hashes and add together values of the same keys...
 
;; feels like I need another map or reduce...

;; I want to merge, no not merge. But merge with a function...

;; A little Google later...

(defn summer [result item] (merge-with + result item))
(reduce summer start-map sum-fundings)
;; {:industry_collaborative 12M :research_charity 4M}

;; :)